Industries which cut lumber from logs or chips the logs for papermaking customarily remove the bark prior to other processes. Trees which are dragged along the ground after felling accumulate dirt and sand in the tree bark; this abrasive foreign material is particularly abusive to saw blades and chipping knives. It is therefore standard in most operations to remove the bark containing the gritty material prior to further operations.
One well known means of removing the bark is by rotating bark tools around the advancing log, applying great force to the tool tip as it and where it touches the log. This action tends to destroy the underlying cambium layer and the bark easily falls away. Good examples of such machines are found in patents to Brundell, et al., U.S. Pat. Nos. 2,857,945 and 2,903,028.
In order to accomodate logs of different sizes, and irregularities in the log, the bark tool tips initially are clustered together and then forced apart as the advancing log meets the rotating crescent shaped tools. A tensioning device similar to huge rubber bands by leverage forces the tips towards the center of the circular shaped machine through which the log travels.
Great forces are continually impinging on the bark tools, and almost daily maintenance is normally required. The tips and the supported end of the tool are easily broken or bent.
Heretofore, the bark tools have been crescent shaped at each end; at the tip, and also at the supported end which is attached to a shaft which transmits the torque created by the rubber tensioning device. An analysis of the moment induced in the tool indicates that the moment linearly increases from the tip to the support, but the moment resisting capacity decreases from about midway of the tool down to the support. For example, FIG. 9 of U.S. Pat. No. 2,857,945 and FIG. 19 of U.S. Pat. No. 2,903,028, and FIG. 5 of U.S. Pat. No. b 2,788,034.
The present invention discloses a novel physical arrangement whereby the tool itself, and its connection to the shaft, are designed to provide structural capacity consonant with stress requirements.
Structural capacity in resisting moment induced stresses is illustrated by the formula EQU .sigma.=(M/S) (1)
where .sigma. is stress, M is the moment and S is the section modulus, which for rectangular sections is determined by the formula EQU S=(bd.sup.2 /6) (2)
where b is the thickness of the member transverse to the plane of bending and d is the depth of the member in the plane of bending. Thus if moment increases linearly, stress may be kept constant by increasing the member thickness or depth, or both. My invention increases the depth of the tool from the tip end to the connecting end such that stresses in the tool are relatively constant throughout the tool length. Thus the structural shape governs its moment resisting capacity. The tip end used in my invention is commonly used in the art and has means to debark logs.